The invention relates to a grading plant comprising a self-cleaning screen for lump materials retaining oversize fractions compared with a predetermined material size, the screen being displaceable by means of a moving device backwards from a horizontal sorting position to a vertical discharging position, and forwards back to the sorting position, the screen being guided in tracks with a horizontal portion and a curved downwards directed portion in connection with the horizontal portion.
Such grading plants are utilized to retain oversize fractions of lump materials which are to be carried through stone crushing plants and used e.g. for levelling on building and construction sites.
The operation of crushing plants may from time to time be stopped for long periods by the jamming of too big blocks between the breaker plates. Removing of such blocks may be very work consuming and take a long time during which the production of the crushing plant will be strongly reduced. Thus, a strong demand exists to provide equipment which can sort out oversize fractions of rocks before the materials are put into the crushing plant, thereby ensuring continuous operation.
At construction sites where lump materials are taken out for later use as filling materials for levelling, road building, etc., it is desireable to be able to carry out a safe construction of such base layers by as far as possible using approximately homogeneous materials ensuring stability. By retaining undesired fractions, this may to a great extent be achieved.
U.S. Pat. Nos. 2,396,954, 2,690,840 and 3,402,816 disclose grading plants with screens pivoted upwards for discharging retained fractions. These patent specifications disclose a screen which may be pivoted about an axis at one end. This means that the distance from the place where the sorted fraction falls down to the place where the retained fraction is discharged is comparatively small. Discharging of the retained fraction demands by certain solutions a high amount of force, depending on the quantity of the fraction which is to be discharged. Discharging is effected by means of engine and mechanical transmissions or hydraulically.
U.S. Pat. No. 1,685,823 discloses a screen which is movable on wheels along a track containing a horizontal portion and a curved portion, whereby the screen is pushed until the wheels are in the curved part, whereby the retained fraction is discharged and a spring is tensioned, which spring thereupon moves the screen back to the starting position. As the entire screen with its relatively large weight is pivoted so that the center of gravity is displaced a considerable vertical distance downwards, a disproportional strong spring is required to absorb sufficient energy as to move the screen back to the correct starting position.
Norwegian Pat. No. 117 159 discloses a screen with two pairs of wheels where the pairs are guided in straight tracks forming an acute angle relative to each other. This solution gives an uncontrolled discharging of the screen, as the motion of the screen towards the discharging position starts when the weight on the protruding part of the screen exceeds a certain value, whereas the returning motion of the screen starts when the weight balance upon discharging of the retained fraction is displaced so much that the center of gravity no longer is located in the protruding part. To start the discharging motion of the screen, the materials preferably have to be put on the protruding part of the screen.
The return motion of the screens according to the two last mentioned patent specifications is not ensured to be executed quite forwards to the sorting position.